Heater control circuits



Feb. 3, 1959 A. A. OBERMAIER 2,372,555

HEATER CONTROL CIRCUITS Filed July 25, 1957 4 Sheets-Sheet 1 Feb. 3,1959 A. A. OBERMAIER HEATER CONTROL CIRCUITS 4 Sheets-Sheet 2 Filed July23, 1957 Feb. 3, 1959 A. A. OBERMAIER 2,872,556

HEATER CONTROL CIRCUITS Filed July 25, 1957 4 Sheets-Sheet 3 Feb. 3,1959 A. A. QBERMAIER HEATER CONTROL CIRCUITS 4 Sheets-Sheet 4 Filed July23, 1957 United States Patent O HEATER 'CONTROL `CIRCUITS Alfred A'.Obermaier, Park Ridge, Ill., assignor to Iilinois Testing Laboratories,Inc., Chicago, lll., a corporation of Illinois Application July 23,1957, Serial No. 673,647 Claims. (Cl. 219-20) The present inventionrelates to heater control circuits, and is particularly concerned withimprovements in heater controlling circuits of the type employing asaturable iron core reactor such as that employed in the U. S. patent toVictor W. Breitenstein, No. 2,341,526, issued February l5, 1944, thedisclosure of which is hereby incorporated herein by reference thereto.

One of the objects of the invention is the provision of an improvedheater controlling circuit of the class described which has improvedoperating characteristics with equal sensitivity, and which is moreflexible in its application to the control of heaters because it canmore easily be made in a control of the on or olf type or a control ofthe proportioning type.

Another object of the invention is the provision of an improved heatercontrolling circuit in which the circuit n and its component parts canbe manufactured and assembled more economically, and in which the secondreactor stage may be eliminated, and a low cost transistor may beemployed in the second stage.

Another object of the invention is the provision of an improved heatercontrolling circuit utilizing a saturable core reactor in which theoscillating type of operation is employed, and in which the period ofoscillation can be made variable by adjusting the resistance so that theperiod may be a sub-multiple of the normal oscillations as the operationapproaches the control point or, if the period desired is more than thenormal period of the circuit, it may be operated as an off or oncontrol.

Another object of the invention is the provision of an improved circuitofthe class described in which a Zener reference diode is employed forproviding constant potentiometer current, and in which the condenser,which provides current for the relay, is pre-charged by direct current,utilizing a full wave or half wave rectifier for the reason that theoutput of the transistor is not sutiicient to charge the condenserfully.

Another object of the invention is the provision of an improved circuitin which the second reactor stage may be eliminated by the use of atransistor in its place, allowing the rst ferro-resonant reactor stageto be operated as a sub-harmonic oscillator, as the control point isreached, so that the controller can be made to operate as a strictlyon-ott control or a modulating type with a variable band width, whichcan be accomplished by varying the R. C. time constant of the relayholding circuit.

Another object of the invention is the elimination 'of the reactor typevoltage rectifier employed in the Breitenstein patent by the use of aZener reference diode as a constant voltage for the potentiometercircuit.

Another object of the invention is the provision of an improved controlcircuit of the class described including a pre-charging circuit for theR. C. network for holding in the relay for the reason that when thecontroller starts oscillating and the control point is reached, theinitial charge to the condenser from the transistor circuit would lll PIC

be too small to hold in the relay, thereby making it necessary topre-charge the condenser.

Other objects and advantages of the invention will be apparent from thefollowing description and the accompanying drawings, in which similarcharacters of reference indicate similar parts throughout the severalviews.

Referring to the four sheets of drawings accompanying the specification,

Fig. l is a wiring diagram of the control circuit;

Fig. 2 is a modified wiring diagram in which the resistance thermometeris employed instead of a thermocouple;

Fig. 3 is a modified wiring diagram in which the relay is eliminated,and in which a 50 v. a. reactor controls a 25 kw. reactor that controlsthe furnace heater;

Fig. 4 is a diagrammatic view of the type of saturable core reactoremployed;

Fig. 5 is a diagram showing the Zener diode characteristics. A Zenerreference diode is dehned as a silicon junction diode used at its Zenerbreak-down point.

Referring to Fig. l, this is a wiring diagram of an amplifying circuitfor a low to medium impedance input, such as a thermocouple. The circuitpreferably includes a saturable iron core reactor in its iirst stage,generally indicated `by Li, the structure of which is alsodiagramrnatically shown in Fig. 4.

The reactor is of the type having a closed magnetic circuit, andpreferably includes three legs in its magnetic circuit for receivingcoils, such as the core 20 of Fig. 4, having the legs 2l, 22, 23 joinedby the side portions 24 and 25 of the core.

The central leg 22 is preferably of twice the crosssectional area ofeither one of the end legs 21 and 23, so

that the cross-section of the central leg is equal to the sum of thecross-sections of the end legs.

The reactor is provided with balanced windings, including the windings26, 27 for alternating current on the legs 21 and 23, connected inseries, and in the same direction of rotation, considering themagneto-motive turns of the coils as producing a iiux extending in thesame direction about the outer circuit of the core, as shown by thearrows 28.

The terminals of these balanced windings 26 and 27 for alternatingcurrent may be indicated by a and b, for association for similarterminals in other circuits.

The central leg 22 is provided with a winding 42 which has the terminalsindicated by c and d; and the winding 42 is made in such a direction asto produce a flux in the direction of the arrows 3, pointing upward inFig. 4.

The two outer legs are surrounded by the alternating current orreactance coils 26, 27, previously mentioned, connected in series insuch a way that the alternating current iiux produced by these two coilsin the center leg oppose or cancel each other, making the center legiiuxless in so far as alternating current is concerned.

The center leg holds the direction current or Ho-coil, and notransformer action takes place between this coil and the alternatingcurrent coils as long as no direction current is super-imposed.

Reference may be had to the said prior patent for the size of the partsof the core in relation to the ampere turns and for one example of thesizes and structures which may be employed.

All of the constants given herein are merely exemplary of those whichmay be employed in one exemplary circuit.

Referring to Fig. l, this is a diagram of the control circuit whichembodies improvements over the said prior patent, which give an improvedperformance and may be constructed more economically due to the use of alesser number of components, which may also be of lower cost, and whichare adapted to accomplish substantially the same general results, butwith improved performance.

The basic ferro-resonant control circuit of said prior patent isemployed with the following changes. For example, a three legged reactorL1 is employed, and indicated at 41 'm Fig. 1, in series with acondenser 44 and a full wave copper oxide rectifier 45. The circuitincludes a rectier 45 connected to the energizing winding 62 at thepoints C and D.

l. The potentiometer input circuit of Fig. l is essentially the same assaid prior patent, and consists of a network including thethermocouple59, a resistance 60, avwinding 42 on the reactor 41, a potentiometer 58,and a resistance 57. L Referring again to the energizing circuit for thereactor, this includes the transformer 100, having a magnetic core 101and a primary winding 102, the terminals 103 and 104 of which areadapted to be connected to a 120 volt 60 cycle lighting current circuit.The transformer 100 has two secondary windings, one of which, indicatedat 62, has terminals C and D, which may be connected by conductors 105and 106 to terminals bearing similar legends, and indicated at 107 and108.

' Terminal 108 is a primary terminal for the .reactor 41. The primarywindings 26 and 27 of the reactor are 'energized by alternating currentandare connected in series with each other and with the terminal 108.The other terminal of the reactor A. C. windings 26 and 27 is indicatedat 109 and is connected by conductor 110 to one terminal of thecondenser 44, which may, for example, be a 200 volt condenser having acapacity of .25 mfd. The other terminal of the condenser 44 is connectedby conductor 111 to one of the terminals 112 ofthe rectifier circuit,the opposite terminal of which is indicated at 113.

-V AThe rectier 45 includes four conductors 114, 115, 116, and 117connected in the nature o f a bridge, and including the copper oxiderectifier discs in such manner that the two opposite terminals orcorners of the bridge 118 and 119 are the plus and the minus terminalsof the rectifier 45, as indicated by the legends.

The alternating current input to the rectier is by wayofconductors 111and 107; and the direct current output is from the terminals 118 and 119by conductors 120 and 121. Referring again to Athe potentiometer inputcircuit, which includes the network 59, 60, 42, 58, and 57, the elementsof this circuit are described in more detail, as follows.

f The central leg of the reactor L1 is provided with the control winding42 and with the bias winding 43. The control winding 42 is connected byconductor 122 to the resistor 60, which has its other terminal connectedby conductor 123 to the positive or plus terminal 124 of thermocouple59. The negative terminal 125 of thermocouple 59 is connected byconductor 126 to one terminal of the potentiometer 58, which may, forexample, be a 40 ohm potentiometer.

The thermocouple 59 consists of a pair of dissimilar metallic conductors127 and 128 joined at a hot junction 129 and having their cold junctionlocated at 124 or 125 spaced from the hot junction 129.

The potentiometer 58 may consist of a series of windings of resistancewire engaged by a slide wire contactor 130, which is connected byconductor 131 to the other terminal of the controlcoil 42. The contactor130 is adjustable to vary the amount of resistance included in thecircuit by the potentiometer 58 and contactor 130.

A series resistor 57 is connected to the right terminal ofpotentiometer-58 and is connected to the terminal 131 bearing the legendR of the bias coil circuit.

The bias coil 43 for the reactor may have one terminal connected byconductor Y132, 133 to the terminal 134 of the potentiometer 58. Theother terminal of the bias coil 43 is connected by conductor 135 to aslide Wire contact 136 engaging a resistor 61, which may, for example be1500 ohms,'and which is in series with resistor 62 of 112 ohms, forexample, the other terminal of which is connected to the terminal R or131.

The terminal 131 is-connected to a Zener diode 56, which has its otherterminal connected to conductor 133 at a terminal T.

The slide wire contactor 136 is connected by conductor v137 to a movablecontact 138 engaginga-'xed contact 139, which is connected'by conductor140 to the terminal 141 located between the resistors 61 and 6 2.

The purpose of the' switch 138, 139 is as-- follows:

All the switchcontacts -shown in Y-the diagram are operated by therelay48, and the switch 138, 139 serves as an anticipating control, beingnormally open and being closed when the relay 48 is energized.

Whenthe relay 48 is 'energized by discharge of the `condenser 54, theswitchis closed at Contact 139 by switch arm 138. This causes the systemto operate as an anticipating control, which turns off the heater beforethe .desired temperature is reached, to prevent over-shooting thattemperature. The circuit is then turned on again for a short time andolif again by the control to supply heat in successively smallerincrements until the desired temperature is reached.

i For example, assume the control is calibrated for a certaintemperature on the temperature dial. When the temperature reaches thatpoint, such as 300 degrees F., 4arr equal and opposite controll signalmay be applied to the thermocouple circuit to turn oif the heater at 300degrees. This would permitthe -heater to over-shoot thedesiredftemperature of 300 degreeslf' To avoidover-shooting,th'e'resistor 61 is adjusted to such a value that the heater will beturned off at 250 degrees F., when the Vpointer is still at300-deg1'eesl?. The switch 138 short circuits theV resistor 61, whencontact is closed at 139 by relay 48; butwhen the circuit is not closedat 139 by relay 48, the resistor 61 is in circuit. The circuitis on truecontrol point when the switch 138, 139 is closed; but circuit is onreduced anticipating 250 degree F. control point when` resistance 61 isin circuit and switch 138, 1 39 is open. When relay 4S is actuated,resistor 61 which is in circuit to stop heat at 250 degrees, is shortedby closing switch 138, 139.

When furnace heater is cut ot 'at 2 50 degrees by rising temperature atthe thermocouple, resistor 61 is cut out, setting the controly back tothe true control point. Then the furnace not having reached`300 degreesF., it is turned on again and' off again to supply Ysuccessively smallerincrements of h'eatuntil it` reaches 30() degrees F. Whenever thecontrol' current reacheszero current or balance, the heater is turnedoifand then on and oli by small increments of heat input until'thetemperature of 300 degrees F. is reached, Thus the anticipating controlavoids over-shooting of the desired temperature setitng. l

The purpose of the circuit which includes the resistors 61 and 62, Zenerdiode 56,;and biaswinding 43 is to place a constantvoltage on the biascoil 43 of the reactor 47 to provide a constant voltage for thepotentiometer input by Ameans of the Zener reference silicon diode 56.This is connected to the circuit at the terminals indicated by legends Rand T; and theregulation of this unit is dependent onvitsnonlinear-characteristics whena. nega-` tive voltage is applied to itsbreak-down point.

In Fig. 1 the terminal 131, bearing legend R, is indicated as plusorpositive; and the terminal indicated by legend T is negative, forapplying the negative voltage to the Zener diode.

Referring to Fig. 5; this is a diagram showing the voltagecharacteristics ofthe Zener diode. Thevertical axis is indicated at thetop by plus I andatthebottom by minus I. The horizontal .axis isindicated at the savante,

left by minus V and at.the rightby plusv V, indicating minus Voltage andplus voltage. The curve Z indicates a curve for a Zener diode, showingtheY resulting voltage of plus or minus, which is brought about byapplying predeterminedplus or minus currents to the diode.

l It will be noted that the positive end of the curve Z curves graduallyupward; but the negative end of the curve Z extends horizontally alongthe axis; and 'upon application of a negative voltage of, for example,minus 6 volts, the curve Z extends downward and approaches a straightline characteristic, indicating that in this regulating region, as shownby the legend, the voltage applied by the Zener diode to the bias coil43 may be substantially constant at minus 6 volts.

By means of this circuit including the Zener diode, a substantiallyconstant voltage may be applied to the bias coil 43; and the voltageregulation which is eiliected thereby is extremely stable.

The second reactor stage of the circuit of said patent may thus beeliminated and replaced by the common emitter transistor circuitproducing improved results and utilizing less expensive components.

The purpose of the resistance 57 in the potentiometer circuit is toprovide a calibration resistance for setting the range of thepotentiometer circuit.

, The purpose of the xed resistor 62 is to serve as a current limitingresistor, limiting the current to the bias v coil.

The present control circuit differs from the patented circuit in the useof a transistor 145 having the base indicated at E by numeral 146, theemitter indicated at E with its terminal 147, and the collectorindicated at C with its terminal 148.

Various types of transistors may be employed, such as, for example, theGeneral Electric 2N187A or Raytheon CK761.

4The base 146 has its terminal connected vby conductor 121; to the minusterminal 119 of the rectifier 45.l The positive terminal 118 of rectier45 is connected by conductor 120, and conductor 149 to the emitterterminal 147.

The resistor 51 of, for example, 210 ohms is connected across theconductors 121 and 120 as a parallel current bleeder to limit the basecurrent of the transistor. The collector output circuit from terminal148 of the collector C is connected to a series resistor 150, which may,

for example, be 560 ohms, 1 watt, which is connected by conductors 151and 152 to the coil 153 of the relay, which, for example, may have 1000ohms.

. The relay is indicated in its entirety at 48 and has the terminals ofits coil bridged by a condenser 154, which may, for example, be 4 mfd.The conductor 1Z0, extending from rectitier 45, is connected to aresistance 49, which may be of 510 ohms and is also connected at 155 tothe emitter terminal 147 by conductor 149.

Resistor 49- is connected to the terminal 156 of the relay coil 153 byconductor 157. The conductors 151 and 152, extending to relay 48, areconnected to a resistor S, which has its opposite terminal connected` byconductor 159 to conductor 160 leading to the positive or plus terminalof a condenser 54, which, for example, may be 4,000 mfd.

The negative terminal of condenser 54 is connected by conductor 161 to amovable switch arm 162 adapted to engage a contact 164. Contact 164 isconnected to terminal 15d of the relay and is adapted to be used toapply the direct current charge, which is stored by condenser` -54, tothe relay 48 to provide suicient current to keep the relay coilenergized over a series of pulses from the control circuit.

The charging circuit ofFig. 1 includes the secondary 4,6. ofthetransformer 100, which has one terminal 170 connected by conductor171 to the half wave rectifier 6 172 at its negative side, the plusbeing connected by conductor' 173v to the conductors 159 and 160 at 174.

The switch arm 162 is adapted to engage two contacts numbered 164 and175. Contact 175 is connected by conductor 176 to an intermediate tap177 on the transformer secondary 46 to yutilize nine volts of thistransformers secondary for impressing the voltage on the half waverectifier to produce a direct current for charging the condenser 54.

The entire winding 46 is connected by conductor 170 to one of theterminals 178 of the rectifier 52; and the other end of the secondary 46is connected by conductor 179 to the opposite terminal 180 of rectifier52.

This rectier is used for impressing suitable D. C. voltage on thetransistor and the relay coil 153. Conductor 181 connects the minusterminal 182 of rectifier 52 to the terminal 156 of the relay coil,which is connected to contact 164 and to resistance 49. This terminal152 of the rectier is also connected by conductor 183 to the inputcircuit T, that is, to the conductor 133.

The plus terminal 184 of the rectiier 52 is connected by conductor 185to the terminal 136 of resistor 55, which may, for example, be 2700ohms, and which in turn is connected to terminal 131, bearing legend R,in the input circuit. 1t is the rectier 52 that provides energizationfor the biasing coil 43, maintaining constant voltage by means of theZener diode 56.

Another conductor 137 extends from terminal 186 to terminal 138, whereit is connected to the positive side of the condenser 50, which is a 25volt, 50 mfd. condenser. The positive side of this condenser isconnected by means of conductor 139 to conductor 187; and the negativeside of this condenser 50 is connected by conductor to conductor 181.

Thus condenser 5t) is bridged across the resistor 49, and the condeusers154 and 5d are used for providing a matched impedance in the collectorcircuit of the transistor 145.

The operation of the heater controlling circuit described is as follows:

In the said patent there are described four modes of operation oroperating conditions, Nos. l, 2, 3, and 4, on page 5 of the patent.

The present circuit is adapted to be operated in the iirstferro-resonant reactor stage, involving reactor 41, as a sub-harmonicoscillator. The operating conditions of this circuit are those ofcondition No. 3, called Case No. 3 in the patent, in which the reactoris biased with a magnetizing force of a value which is slightly lessthan the value indicated at M in Fig. 9 of the patent.

Since the Breitenstein patent is already incorporated by referencethereto in column 1 hereof, the operating conditions No. 3 of the patentare explained with regard to Fig. 9 of the patent.

As stated in the patent, page 5, column l, line 34, operating conditionsNo. 2 are attained by operating the' circuit at a higher alternatingvoltage, which causes the current to jump up to the point P in Fig. 9;but it does not break out in oscillations; and the current value staysthere Ias long `as the current saturation plus the controlling incrementremains the same.

For operating conditions No. 3 at the point P on the curve, the circuitis responsive only to an impulse of incremental saturation current andwill be unaffected by a postive increment of saturation current, asstated in the patent, page 5, column 2, lines 8-10.

This brings about the following operating conditions in the presentcircuit.

This causes the current to stay at the point O on the curve of Fig. 9,and is the condition in which the present circuit is when thepotentiometer isy so adjusted with respect to the thermocouple to reducethe input current to zero.

When an additional increment of magnetizing force is applied in the samesense as the biasing magnetizing force, under conditions of Case 3, thecurrent jumps from the point O on the curve in Fig. 9 to the point P andstays there until it is opposed byan impulse of the same magnitude, butof opposing polarity, when it will return to the point O. Y

Thus the application of a signal from the thermocouple causes a verylarge change in the amount of current in the output circuit of thereactor 41; and this change is impressed upon the rectifier 45 andapplied to the input circuit of the transistor 145.

l, The transistor replaces the second reactor-stage of the said patentand operates similarly, inasmuch as when no current flows in theferro-resonant rst stage, the collector current energizing the relaycoil is zero. When an oscillation current is produced by the firstferro-resonant stage and impressed on the collector circuit, thiscollector circuit starts conducting and thereby energizing the relay.The Weak current pulses of the ferro-resonant rst stage are in the orderof 0.5 milliampere; and the amplification by the transistor circuitproduces a 20 milliampere pulse through the relay coil. This issufficient to actuate the relay; and the relay moves the contact arm 162from the contact 175 to the contact 164.

The purpose of this is to provide the RC timing network Vwith suchenergization, with one-half of the transformer winding 46, that therelay coil may he kept ener gized for a series of pulses from thecontrol circuit. The reason for applying external power to the RCnetwork is that under initial conditions, when the potentiometer inputis balanced, no pulses are produced in the ferroresonant circuit; but ifthe input is unbalanced, due to an error signal, the circuit startsoscillating, so that the relay would 'also pulse.

The object of an RC time delay network is to hold the relay in for agiven number of pulses so that there is no excessive wear on the localcontacts. Due to the short duration of the pulses of current in therelay, it is neces'- sary to pre-charge the condenser from an externalpower source, las previously described.

. This charge, which is applied to the condenser 54 by means of theupper half of the winding 46 and the half wave rectifier 172, may chargethe condenser when the larm 162 engages contact 175. The relay 48 havingmoved the arm 162 into engagement with the contact 164, due to thepulses in the relay coil, the condenser 54 may then discharge throughthe relay coil.

This aloWs the RC time to hold the relay energized so thatrsubsequentpulses from the control circuit can build up sufficient charge to keepthe relay energized during the period the error signal persists.

Another important feature of the operation is that the winding 46connected to rectifier 52 supplies both the collector current for thetransistor 146 and the voltage regulated bias current for coil 43 andthe potentiometer input.

This voltage must be maintained constant; and this is done by means ofthe Zener reference silicon diode 56, which is a commercially availablevoltage regulated device connected between points R and T. Theregulation of this unit is dependent on its nonlinear characteristics.

, The voltage current characteristics of such a diode are shown in Fig.5, where negative voltage is applied to the diode at the left.

Negative voltages from zero to minus 6 volts produce practically zerocurrent, but adjacent minus 6 volts on the curve Z, the Zener break-downoccurs; and the curve Z extends downward slightly.

This is the regulating region of the Zener curve; this type of voltageregulation can be made extremely stable in the regulating region. Thecurrent may vary greatly, While the voltage remains substantiallyconstant.

Y .The elimination of the second reactor stage of the patented device,and performance of its functions by a transistor, `allows the firstferro-resonant reactor stage to be operated as a sub-harmonic oscillatoras the control point is reached.

The period of oscillation can be varied by means of the resistance 61 sothat the period'can be a sub-multiple of the oscillations as thecontroller approaches the control point. If the resistance is soadjusted that the time of one oscillation is more than the period of therelay, then the circuit operates on and off.

Another important feature of the invention is the use of the Zenerreference'diode, which is usedfor the constant voltage potentiometercircuit. This element of the circuit costs less than the reactor stagedescribed in said patent for maintaining constant potentiometer currentand also accomplishes an extremely stable regulation of thepotentiometer current.

Another important feature is the arrangement .of the charging condenser`and the circuits by means of which the` condenser is pre-charged by theD. C. current which may be produced by a full Wave or half Waverectifier.-

The output of the transistor is not sufficient to charge the condenser;but in the present circuit the condenser is charged from a chargingcircuit and then discharged through the relay for holding in the relay.Such a precharging circuit for the RC network is necessary for holdingin the relay. Y

When the controller starts oscillatingA as the control 'point isreached, the initial charge to the condenser from the transistor wouldbetoo small to hold in the relay; and therefore it is necessary topre-charge the condenser.

Referring to Fig. 2, this is a modification in Whicha resistancethermometer is employed as the temperature responsive element instead ofa thermocouple. This circuit utilizes the same 'elements as Fig. l,beginning at x and y in Fig. l, but substitutes the resistancethermometer bridge. for the thermocouple and potentiometer of Fig. l.

All other parts of the circuit may be the same in Fig. 1, beginning atthe points R and T on opposite sides of the Zener diode in Fig. l; andall of the parts of the circuit to the right of R and T employ the samereference numbers and operate in the same way.

Beginning at the point x, which is one of the terminals of the controlwinding 42, a conductor 200 extends to the point x or terminal 201 onthe resistance bridge, indicated in its entirety by No. 202. This bridgeincludes four resistances, indicated at 70, 71, 72, 74,.and thetemperature .responsive resistance 69.

The terminals of the bridge are 201, 203, 204, and 205. The temperatureresponsive resistance 6 9.is that element which is subjected tothetemperature produced by the furnace to be controlled; and this elementis of such material that it changes its electrical resistance withchange of temperature, this being the usual operation Iof a resistancethermometer.v

The resistances 70 and 71 are connected in series be tween the terminals201 and 203; and the terminal 203 is connected by the conductor 206 to aslide wire contactor 207 engaging the coiled resistance 71 and shortcircuiting part of this resistance to vary the voltage drop in this partof the bridge and adjust the output current from the resistancethermometer to zero.

The resistance 72 is connected between the terminals 203 and 204; andthe resistance 74 is connected between the terminals 204 and 205. Thesignal output from this resistance thermometer is at x and y orterminals 201 and 204 and is applied to the coil 42.

The resistance thermometer is energized from Vthe transformer 'by meansof the rectifier 52, which applies appropriate voltages to thisresistance thermometer circuit at the points R and T. The point R isconnected by conductor 208 through resistance 73 to the terminal 203 ofthe bridge. The point T is connected to the terminal 205 of the bridgeby conductorV 133. The operation of this type of circuitislsubstantially the same as that described in Fig. 1 except thatthe 9signals for turning the heater on and off are generated by theresistance thermometer bridge 202.

Referring to Fig. 3, this is a circuit showing a modication similar toFig. 1 except that the relay coil is replaced or used as a controlwinding for a large saturable core reactor.

In this case there is no charging circuit for the relay condenser, sincethere are no contacts employed. Instead of the relay coil, a controlcoil 153a is employed and connected in the same manner previouslydescribed and bridged by means of a condenser 154.

The coil 153:1 is the control winding of a large saturable core reactor,indicated at 210. This saturable core reactor may have the usual biasingcoils (not shown) for bringing the reactor up to the point adjacentsaturation. Reactor 210 also has two alternating current windings 211and 212 in series; and the A. C. output from these windings is furtherrectied by a rectifier 213.

The rectifier 213 may be connected by conductors 214 and 215 to anenergizing circuit having 120 volts A. C. between conductors 215 and216. The output from the rectiiier 213 may be fed to a very largereactor 217 by means of 4coil 218; and the large reactor 217 may have A.C. coils 219 and 220 energized from 110 volts A. C. and connected inseries with the heater 221 for a large furnace.

The advantage of such a system is a stepless controll unit with no localcontacts. Each of the two reactors 210 and 217 would be provided withthe usual bias coils for raising their saturation to a predeterminedpoint.

It will thus be observed that I have invented improved heatercontrolling circuits utilizing the tirst reactor stage of the patentmentioned and eliminating the second reactor stage by the use of atransistor, allowing the first ferro-resonant reactor stage to beoperated as a sub-harmonic oscillator, which can be made to operate as avmodulating type by varying the amount of.

resistor 61.

The expensive reactor type voltage regulator of said prior patent hasbeen eliminated by the use of a Zener diode for providing a constantvoltage source for the potentiometer circuit. The entire circuit may beconstructed more economically, as it employs elements which are lower incost, but which perform better and with equal sensitivity, and provide amore flexible system that can readily be adapted for on and off controlor a proportioning type of control.

While I have illustrated a preferred embodiment of my invention, manymodications may be made Without departing from the spirit of theinvention, and I do not wish to be limited to the precise details ofconstruction A the :scope of the appended claims.

Having thus described my inventtion, what I claim as new and desire to.secure by Letters Patent of the United States is:

l. A heater control system comprising a heater, a temperature responsiveelement subjected to the temperatures produced by the heater, asaturable iron core reactor having windings energized by alternatingcurrent for producing a predetermined flux in the core reactor, saidcore reactor having a biasing winding, a control winding, a rectifierenergized from the output of said core reactor, and a transistor havingits base and emitter energized by said rectier, said transistor having acollector connected to the winding of a control relay, the output ofsaid transistor being normally insufficient to actuate said relay, and acondenser connected across the coil of said relay and connected to arectifier to be charged by the output circuit sufliciently so that whenit discharges through the relay coil its output is sufficient to actuatethe relay and hold the relay energized during the period the controlsignal persists.

2. A heater control system comprising a saturable core reactor having aneasily saturable core and a pair of balanced windings connected inseries with a capacitive react-ance to provide a resonant circuit, asource of'alter nating current for energizing said balanced windings, abias coil on said reactor, and a Zener diode circuit maintainingconstant voltage on said bias coil, an input coil on said reactor, and atemperature responsive member and circuit having means for adjusting itsinput current to zero, and an output circuit including said balancedwindings, a rectifier, a transistor and circuit, and a relay coilcontrolling a heater, said transistor amplifying the weak currentimpulses in the reactor output circuit to provide a suiiicient currentto actuate the relay, said temperature responsive member comprising athermocouple and its means for adjusting its input current comprising apotentiometer.

3. A heater' control system comprising a saturable core reactor havingan easily saturable core and a pair of balanced windings connected inseries with a capacitive reactance to provide a resonant circuit, `asource of alternating current for energizing said balanced windings, abias coil on said reactor, and a Zener diode circuit maintainingconstant voltage on said bias coil, an input coil on said reactor, and atemperature responsive member and circuit having means for adjusting itsinput current to zero, and an output circuit including said balancedwindings, a rectiiier, a transistor and circuit, and a relay coilcontrolling a heater, said transistor amplifying the weak currentimpulses in the reactor output circuit to provide a suiicient current toactuate the relay, said temperature responsive member comprising aresistance thermometer, the resistance of which is one of fourresistances forming a bridge, and the means for adjusting the inputcurrent to zero comprises a slide wire contact and conductor forshorting out part of one of said four resistors.

4. A heater control system comprising a saturable core reactor having aneasily saturable core and a pair of balanced windings connected inseries with a capacitive reactance to provide a resonant circuit, asource of alternating current for energizing said balanced windings, abias coil on said reactor, and a Zener diode circuit maintainingconstant voltage on said bias coil, an input coil on said reactor, and atemperature responsive member and circuit having means for adjusting itsinput current to zero, and an output circuit including said balancedwindings, a rectiiier, a transistorand circuit, and a relay coiicontrolling a heater, said transistor amplifying the weal; currentimpulses in the reactor output circuit to provide a sutcient current toactuate the relay, said output circuit to the transistor including aresistance connected across the conductors extending to the base and tothe emitter ot the transistor for limiting the 'oase current thereof.

5. A heater control system comprising a saturable core reactor having aneasily saturable core and a pair of balanced windings connected inseries with a capacitive reactance to provide a resonant circuit, asource of alternating current for energizing said balanced windings, abias coii on said reactor, and a Zener diode circuit maintainingconstant voltage on said bias coil, an input coil on said reactor, and atemperature responsive member and circuit having means for adjusting itsinput current to zero, and an output circuit including said balancedwindings, a rectifier, a transistor and circuit, and a relay coilcontrolling a heater, said transistor amplifying the weak currentimpulses in the reactor output circuit to provide a sufficient currentto actuate the relay, and an energizing transformer having a secondarywinding connected to said balanced reactor windings, the secondarywindings being connected to a rectiiier to supply direct current to aresistor in the relay circuit.

6. A heater control system comprising a saturable core reactor having aneasily saturable core and a pair of balanced windings connected inseries with a capacitive reactance to provide a resonant circuit, asource of alterestranee navting current for energizing said balancedwindings, a bias coil on said reactor, and a Zener diode circuitmaintaining constant voltage on said bias coil, an input coil onV saidreactor, and a temperature responsive member and. circuit having meansfor adjusting its input current tozero, and an output circuit includingsaid balanced windings, a rectifier, a transistor and circuit, and arelay coil controlling a heater, said transistor amplifying the weakcurrent impulses in the reactor output circuit to provide a suicientcurrent to actuate the relay, said system including a resistor in serieswith the relay coil, said relay coil and latter resistance each beingbridged by a condenser, said condensers being chosen to give matchedimpedance in the collector circuit of the transistor'.

y 7. A heater control system comprising a saturable core reactor havingan easily saturable core and a pair of balanced windings connected inseries with a capacitive reactance to provide a resonant circuit, asource of alternating current for energizing said balanced windings, abias c'oil on said reactor, and a Zener diode circuit maintainingconstant voltage on said bias coil, an input coil on said reactor, and atemperaturevresponsive member and circuit having means for adjusting itsinput current to zero, and an output circuit including said balancedwindings, a rectifier, a transistor and circuit, and a relay coilcontrolling a heater, said transistor amplifying the weak currentimpulses in the reactor output circuit to provide a sufiicient currentto actuate the relay, said relay coil being connected to a networkincluding a condenser to be charged by a rectifier, and a switchactuated by the relay coil to change the connections from a circuitcharging the lattercondenser to a circuit discharging the condenserthrough the relay coil to hold the relay in over a plurality of currentpulses from the output circuit of the transistor.v 8. A heater controlsystem comprising a saturable core reactor having an easily saturablecore and a pair of balanced windings connected in series with acapacitive reactance t provide a resonant circuit, a source ofalternating current for energizing said balanced windings, a bias coilon said reactor, and a Zener diode circuit maintaining constant voltageon said bias coil, an input coil on said reactor, and a temperatureresponsive member and circuit having means for adjusting its inputcurrent to zero, and an outputrcircuit including said balanced windings,a rectifier, a transistor and circuit, and a relay coil controlling aheater, said transistor amplifying the weak current impulses in thereactor output circuit to provide a sufiicient current to actuate therelay, and an auxil- CIy . Y l2 Y iary circuit controlledby the relayand arranged to' apply additional current to the relay coil to hold therelay in closed position.

9. A heater control system comprising a saturable core reactor having aneasily saturable core and aV pair of balanced windingsrconnected inseries with `a capacitive reactance to provide a resonant circuit, asource of alternating current for energizing said balanced windings, abias coil on said reactor, and a Zener diode circuit maintainingconstant voltage on said bias coil, an input coil on said reactor, and atemperature responsive member and circuit having means for adjusting itsinput current to zero,` and an output circuit including said balancedwindings, a rectifier, a transistor and circuit, and a relay coilcontrolling a heater, 4said transistor amplifying the weak currentimpulses in the reactor output circuitto provide a sufficient current'toa'c'tuate the relay, said relay coil comprising the control winding of alarge saturable core reactor, the alternating current output of which isrectified and controls a large reactor supplying heater current for thefurnace.

10. A heater control system comprising a heater, a temperatureresponsive element subjected to the temperatures produced by the heater,a saturable iron core reactor having windings energized by alternatingcurrent for producing a predetermined flux in the core reactor, saidcore reactor having a biasing winding, a control winding, a rectifierenergized from the output of said core reactor, `and a transistor havingits base and emitter energized by said rectifier, said transistor havinga collector connected to the winding of a control relay, the output ofsaid transistor being normally insufficient toactuate said relay, and acondenser connected across the coil of said relay andconnected to arectifier to be charged by the output 'circuit sufficiently so that whenit discharges through the relay coil its output is suicient to actuatethe relay and hold the relay energized during the period the controlsignal persists, the said control coils including a Zener diode`operating within its regulating region and maintaining a substantiallyconstant voltage for the bias coil.

, References Cited'in the file of this patent 27,810,526 Rogers -Q Oct`22, 1957

